Opposition parties on Tuesday ratcheted up their offensive against Financial Supervisory Service chief Kim Ki-sik, who has been under fire for his overseas trips sponsored by financial institutions while he was a lawmaker.

The main opposition Liberty Korea Party filed a complaint with the prosecution against the embattled FSS chief, claiming taking the costly trips amounted to bribery, abuse of power and the breach of public officials' ethics rules.

The minor Bareunmirae Party called for parliamentary and prosecutorial investigations into Kim, while warning it could also consider seeking an independent counsel probe into his case "if necessary."

Kim has been criticized for the trips paid for by the Korea Exchange, the Korea Institute for International Economic Policy and Woori Bank in 2014 and 2015, when he was a member of the parliamentary financial committee. The institutions were subject to the panel's oversight.

Kim has apologized for the controversy but denied that the institutions had received any preferential treatment in return for the trips. The presidential office Cheong Wa Dae has defended him, saying the trips might appear inappropriate but were "legitimate."

But opposition parties' onslaught showed no signs of abating.

"(The presidential office) has punished the practices by former conservative governments under the name of eliminating accumulated ills, but it has not punished Kim," said Choung Tae-ok, the LKP spokesman, referring to the current government's ongoing anti-corruption campaign.

"It is leniently applying the anti-corruption rule to people from the current government while sternly applying it to those from former governments," he added.

The Bareunmirae Party also questioned Kim's integrity, calling him the thief of the financial watchdog.

"There must be a parliamentary probe into why Cheong Wa Dae is defending him and for what reason it has appointed him," Park Joo-sun, a co-chair of the party, said during a general meeting of party lawmakers.

"Going on a business trip with the money from an institution subject to his committee's oversight is a thing unimaginable for a lawmaker with sound common sense. ... The more Cheong Wa Dae and the president defend him, the greater blowback they will face."

Ahn Cheol-soo, a key member of the party, renewed his calls for President Moon Jae-in to immediately fire Kim and issue a public apology.

"Under the name of citizens, I urge all (presidential) officials involved in the personnel vetting process to step down,"

he said during a press conference.

The ruling Democratic Party has dismissed the opposition bloc's calls for Kim's resignation as a political offensive ahead of the local elections slated for June.(Yonhap)